1. Field of the Invention
The invention relates to data communications systems. More particularly, it relates to an alternative international long distance ISDN service for certain applications such as dial-up video conferencing.
2. Description of the Related Art
Integrated Serves Digital Network (ISDN) is a public switched narrowband digital telecommunications service that is available in nearly all developed countries throughout the world. The service is provided on digital telephone switching equipment in 64 kbps duplex channels, and uses conventional copper local distribution plant and digital inter-office facilities for access and transmission. Two interface arrangements are available for user access: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). The BRI consists of two 64 kbps channels, referred to as “B” channels, and a 16 kbps packet switched channel for signaling. This packet switched channel, known as the “D” channel, provides a common channel signaling facility for call setup and control of the B channels. The PRI is configured in two versions depending on the transmission standards employed in the country in which it is located. In North America and some Asian countries the PRI is based on the T1 standard that provides twenty-three 64 kbps B-channels and one 64 kbps D-channel on a 1.544 Mbps facility. European, Latin American and most Asian countries use the E1 standard and the PRI consists of thirty 64 kbps B-channels and one 64 kbps D-channel on a 2.048 Mbps facility. In all cases the B-channels terminate on the central office switch (CO) of the local service provider and are switched and controlled by the user by use of the D-channel. ISDN services are described in ITU Recommendation I.120 (03/93)—Integrated Services Digital Networks (ISDNs), which is incorporated by reference herein for all purposes.
Local service providers offer ISDN services to the general public as an alternative to Plain Old Telephone Service (POTS). ISDN offers enhanced telephone calling features that are not available on the older analog POTS services and even some new data services. For example, a user with a single BRI can set up two simultaneous 64 kbps data calls and then combine them in the user's terminal equipment to provide a single 128 kbps service. Such configuration provides more than twice the maximum data rate that is possible using a single analog POTS line equipped with a modem. ISDN is a superior service for data transmission, which has led to the development of applications that take advantage of that characteristic, such as dial-up videoconferencing.
Videoconferencing technology has evolved over the years to the extent that it can compress the video and audio signals to a level that can be transmitted over relatively narrow bandwidths. For example, the uncompressed digital bandwidth of a standard TV signal is in the range of 270 Mbps. Using data compression techniques, such signals can be reduced as low as 128 kbps for applications that can accept limited image quality and are not required to handle rapid motion. This fits well into a single ISDN BRI, and makes possible dial-up videoconferencing using readily available public switched telecommunications services. If a user wants higher quality, this compression equipment can be configured to provide higher bandwidths in increments of 64 kbps. ISDN can provide the necessary switched 64 kbps channels using multiple BRIs or a PRI. A common configuration operates at 384 kbps and uses three BRIs to derive six 64 kbps B-channels; and such a configuration provides sufficient quality for most videoconferencing applications.
Video codecs for use on ISDN facilities are designed to conform to a standard published by the International Telecommunications Union, this standard is designated ITU Recommendation H.320 (05/99)—Narrow-band Visual Telephone Systems and Terminal Equipment, which is incorporated by reference herein for all purposes. Any H.320 video codec can operate with any other H.320 video codec via B-channel connections through ISDN. Employment of this standard permits codecs made by different manufacturers to engage each other in videoconferencing sessions over ISDN dial-up connections.
However, long distance ISDN services that connect through the transmission facilities of several carriers, particularly international services, often encounter substandard transmission conditions. While these connections may be acceptable for voice communications, they are often unacceptable for data and video applications. In the case of H.320 codecs, the effect is to fail to connect all of the B-channels required for the call resulting in a lower quality video conferencing session. Sometimes long distance ISDN session will setup using all of the B-channels and then drop one or more channels and thereby “down speed” the composite connection during the videoconference session. Accordingly, there is a need for equipment and services to allow videoconferencing applications to operate over long distance ISDN channels, particularly for international dial-up H.320 video conferencing, without suffering the degradation in signal quality that occurs in existing systems.